The present invention relates to radio frequency ("RF") dielectric heaters, and more particularly to an improved RF dielectric heater which eliminates the need to tune the output circuit when changing the output power.
RF dielectric heaters have been used in the past to provide heat in industrial processes for various purposes, such as drying adhesives on a substrate. In envelope manufacturing, for example, it is necessary to dry the glue which is placed on the envelope flap as envelopes being manufactured travel along a production line. In order to provide desired high rates of throughput, a reliable and efficient heater is required to dry the glue. RF dielectric heaters have been found to be particularly useful for this purpose.
RF heating employs a high frequency varying electric field. The substrate to be heated is placed between a pair metal plates, called electrodes, attached to the output circuit of an RF generator. When high frequency voltage is applied to the electrodes, an alternating electric field is set up between them. As a substrate passes through the field, electrical charges within the molecules of the substrate vibrate in proportion to the frequency of the varying electric field. This intense molecular action generates the heat used for dielectric heating.
Generators used for such dielectric heating are known. For example, such a circuit is shown in Article 665 of the National Electrical Code Handbook, which article deals with induction and dielectric heating. However, such past vacuum-tube generators have required the adjustment of the grid and plate circuits in order to achieve desired changes in the output power of the generator. Such adjusting or "tuning" of the grid and plate circuits results in changes in the output operating frequency. Such frequency shifts are extremely undesirable, in that interference with radio communication systems can result. This is particularly troublesome in the case where fire, police, or other emergency service communication bands are interferred with. It would therefore be advantageous to provide an RF generator, for use in dielectric heating, wherein the output power can be varied as needed for an industrial process without changing the operating frequency of the generator.
The present invention relates to such an improved RF dielectric heater.